For the purpose of determining the formation of strata in formations beneath the bottom of the sea, it is the usual practice to tow oil-filled streamer cables behind survey ships carrying complex electronic equipment for recording signals picked up by the hydrophones mounted in the cables and responsive to acoustic pressure waves resulting from local artificially-generated seismic disturbances. The streamers are usually towed at depths ranging from about 20 feet up to about 80 feet, and these streamers are as long as 21/2 miles and have hydrophones therein spaced several feet apart over that length.
Present day hydrophones perform satisfactorily and tend to remain in good condition for a while and until their utility is destroyed, usually either by salt-water contamination occurring when a streamer is cut open or broken allowing sea water to enter, or else by the transducer units being crushed by over-pressure when a streamer becomes accidentally severed from the towing vessel and sinks into deep water. Both of these occurrences are not at all unusual, it being estimated that damage of this sort occurs about once every 6 months to the average streamer in use requiring that it be rebuilt, often at great expense, since the hydrophone units must be individually repaired or replaced. Presently available hydrophones include transducer units in which the various parts are cemented together usually by epoxy plastics which, when exposed to sea water absorb salt. Piezoelectric units are inherently high-impedance devices, and only a small electrical leakage ruins their sensitivity. Boiling them in distilled water only partially restores their original characteristics, and of course, they must be completely disassembled from the streamer before this can be done. Mere flushing of the cable would be ineffective. On the other hand, over-pressure can either crush the transducer unit, or flex it to the point where the piezoelectric wafers are cracked and thereby rendered useless. Spacers are used in prior-art devices to limit the deformation of these units by bottoming against the spacers, and the present invention contemplates their use too, but solves most of the over-pressure problem by mounting the piezoelectric wafers inside strong metal shells in which they are then hermetically sealed.
Typical hydrophones of the prior art which make extensive use of plastic parts and component-to-component seals of epoxy include U.S. Pat. Nos. 3,187,300 to Brate and 3,832,762 to Johnson et al. These are not hermetically sealed transducer units within the meaning of the term as used in the present disclosure where only parts of metal and glass make up the outer surfaces of the transducer units. These parts not only can absorb no salt whatever, but salt-water rolls right off of the glass insulators which are fully cleaned by mere rinsing with distilled water. Another advantage of the metal-glass construction is that these materials are not degraded as time goes on, as is the case with plastic construction causing eventual loss of mechanical integrity by the hydrophone.